This invention relates to an improved metallization system for substrate materials and to a process therefor. More particularly, this invention relates to a metallization system for semiconductor materials.
Semiconductors generally consist of a single crystal element of silicon which has an internal PN junction and has plated metal contacts at surfaces parallel to the PN junction. Electrical connectors are soldered to the metal coating and often one of the connectors is part of the housing which forms an enclosure for the semiconductor device. Metal coatings have been required because solder materials do not wet silicon well enough to permit soldering the connectors directly to the silicon material.
One approach used in the prior art involves an elaborate wet chemical process in which heated wafers are dipped into a nickel plating bath, then a second nickel coating is applied over the first one and then gold plated over the nickel, all by immersing in plating baths with suitable rinsing and drying procedures carried out between the plating steps. In addition to the manufacturing costs inherent in this process, the plating baths often contain contaminants which can degrade the electrical characteristics of the coated devices.
The metallization system chromium-silver-gold is widely used in the semiconductor industry but is characterized by a variety of drawbacks. Thus, there is a certain lack of reproducibility when evaporated onto silicon substrates by conventional evaporation equipment. The silver does not bond strongly with most materials and will peel from the chromium if the chromium becomes oxidized or an adequate interphase is not formed during the evaporation process. Furthermore, at high temperatures e.g., 700.degree.C or above, gold migrates into silicon therefore requiring a thick layer of chromium to function as a barrier.